The present invention relates to a mechanical part that includes at least one circuit for containing a fluid, and to a method for producing such a part.
The present invention is applicable to a broad range of fields such as, for example, mechanical engineering (for example, for the manufacture of cylinder heads), printing (for the production of ink-marking circuits), or other fields. In addition, the present invention preferably, but not exclusively, applies to the field of plastics processing, and more particularly, to the problems posed by the thermal regulation of molding tools (dies or punches).
The thermal regulation of an injection molding tool has the function of extracting thermal energy provided by the molten thermoplastic to the outside of the tool. Such energy is imparted to the thermoplastic by the plasticating screw to allow the thermoplastic to conform to the impression being made. Such energy must then be removed from the thermoplastic so the part can be ejected (without any “distortion” of the molding impression). Such extraction takes place under conditions defined beforehand, during the design of the part and of the tool.
The solution most commonly used to carry out the function of cooling and regulating molding tools is to produce a series of channels in the body of the tool, through which a heat-transfer fluid can circulate. The nature of the fluid depends on the desired average temperature in the tool.
To obtain optimally effective regulating channels, it is necessary for the channels to be able to form a layer facing the part, or which exactly follow the shape of the part, and for such channels to be separated from the part by as thin a wall as possible. In practice, this solution could not be achieved, both for technical reasons and because of the high mechanical stresses generated by the injection molding process.
A similar solution is sometimes obtained by a system of channels having a square cross section, and that approximately follow the shape of the part. This solution is used in special cases and is known to be used only on simple geometrical shapes (mainly on cylindrical punches). Such a solution gives rise to the problem of sealing between the attached parts, resulting in substantial delays and manufacturing costs.
Such channels are most often produced by drilling, which is the least effective but simplest solution. Since the holes can be drilled only in a straight line, an entire series of drilling operations is necessary in order to follow the impression as closely as, possible. The circuit is then formed by using fluid-tight plugs, or even by using external bridging arrangements for difficult cases, which are best avoided to the extent possible due to the risk that the resulting circuits can be crushed or broken while the mold is being handled.
Insufficient cooling can result either due to geometrical precision problems or excessively long cycle times. In the worst cases, this can cause production shutdowns, during which the mold is left open to be regulated by natural convection.
Despite all of these risks of malfunction, this aspect of the tool is often neglected when designing molds for injection molding. The regulating system is very often designed as the last item, and must be placed between the various ejectors, the guiding column, etc. This has been found to be erroneous because this function is the keystone of the injection molding process. The conditions for cooling the part play an essential role in the level of internal stresses in the injection-molded parts and in the crystallinity of the polymer, and therefore, in the aging stability and the mechanical properties of the parts. Consequently, production of the cooling/regulating channels currently represents a major challenge in improving performance in plastics processing.
One solution which has been proposed is disclosed in an article in the journal “Emballages Magazine” entitled “How to Optimize the Molding of Plastics” (January-February 2002, supplement No. 605). The disclosed solution entails the production of a first, prototype mold, the behavior of which is observed and recorded during cooling. A computer then analyzes the data and deduces the dimensions and the positions of pins intended to improve heat exchange. This method leads to the construction of a second mold which is more effective than the first mold, and which includes a set of pins placed in accordance with a design established by the computer. Such a solution is time-consuming and requires prior experimentation.
Another solution which has been proposed is disclosed in International Publication No. WO 02/22341. The disclosed solution places a tubular insert provided with radially disposed pins inside a parison, in order to increase the heat exchange. The application of this solution is limited, and complicated to implement.
The object of the present invention is to alleviate the aforementioned drawbacks of the prior art and to provide an entirely novel method for designing and manufacturing the tool and its fluid transport circuit.